Process for utilizing the heat in the oxidation of ammonia to nitric acid under pressure



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PROCESS FOR UTILIZING THE HEAT IN THE OXIDATION OF AMMONIA TO NITRIC ACID UNDER PRESSURE Filed June 19, 1954 A//rff/c /46/0 @auf-y' Hwa/P5595 Meu ne' 8 5@ 7 /6 @/mf/mm www? ,Q7/Wp /12 V50?? mr/ffm 850 f6. v 1 /y/y Patented Feb. 18, 1936 i UNITED STATES PATENT oFFlcE PROCESS FOR UTILIZING THE HEAT IN THE OXIDATION OF AMMONIA T NITRIC ACID UNDER PRESSURE V Application June 19, 1934, Serial No. 731,254 In Germany August 4, 1933 4 Claims.

The-subject of the present invention is an improvement of the process described in the U. S.

Patent No. 1,954,317.

In the process described in said patent a mixture of ammonia; andair under pressure is burned and the hot products of combustion comprising the oxides of nitrogen are then cooled in heat exchangers and in a condenser and then passed through absorption apparatus in which the nitrogen oxides are absorbed to form nitric acid, the last traces of lnitrogen oxides being removed in an alkaline'wash. The residual gases still under pressure, but cooled to approximately atmospheric temperature," are then reheated in heat exchangers and used to drive a compression machine for compressing air supplied to the combustion 'of the ammonia. Inthis reheating of the residual gases preparatory to their utilization for driving the compression machine, they are first brought into heat interchanging relation to the exhaust gases from the compression machine in a heat exchanger identified by the numeral 9 inthe patent. Thence they are further heated by heat taken `directly from the products of com- 25 bustion of the ammonia 'and air. 'Ihe heated Ygases are then usedto drive the compression machine and the heat again abstracted from the exhaust gases and returned to a fresh quantity of residual gases being supplied to the compression machine. Y

The heat exchanger 9 utilized for this purpose must have very great heating surfaces, inasmuch as the heating gas is expanded and therefore-its coefficient of heat transmission to the wall of the pipe is small. To increase the coeicient of heat transmission by increasing the velocity of the gases would require an undesirable increase in the back pressure on the expansion engine.

Although the process of Patent 1,954,317 greatly increased the eiciency of the utilization of heat in the oxidation of ammonia over processes theretofore known, it did not recover and utilize heat given up in the condenser immediately preceding the absorption of the nitric acid and also quantities of heat still remaining in the exhaust gases.

In my present invention I provide a process in which additional heat is recovered and utilized, making it possible to decrease the heating surface required in certain of the heat exchangers and, therefore, reducing the cost of these eX- changers which are generally made of expensive corrosion resisting metals.

In my invention I utilize heat abstracted in the condenser by causing the residual gases from the absorber to pass into contact with the heated water from the condenser in a suitableV saturator or scrubber, whereby some of the water is evaporated and added to the volume of the residual gases. The volume of the residual gases increased by the added water vapor or steam then passes into and through the respective heat eX- changers and is supplied to the compression mechanism to drive the latter.

It has been proved that the cooling water flowing from the condenser 6 can be obtained suciently hot, to produce the said direct preheating of the residual gases (for example up to.90 C.).

For example, the residual gases are sufficiently saturated with steam at a working pressure of 8 atmospheres and at a temperature of C. for their volume to be increased by about 10%.

This saturation with steam allows the entering temperature of the'residual gases in the expansion engine to be lowered, and this is an advantage as regards the expansion engine, without diminishing the yield of the latter owing to the increase in the volume of the gases.

The exhaust gases from the exchanger 9 can also be used for the preparation of the hot water, so that smaller-.heating surfaces are required for the transfer of heat from the exhaust gases from the compression machine to the residual gases augmented by the added moisture or steam prior to the nal heating and supply of the latter to the compression machine.

Only a part of the condenser 6 of Patent 1,954,317 need likewise be used for the heating of the water, so that the water between thepart of the condenser and the saturator is kept in a closed circulation.

In the case lof such a circulation, instead of water, acids, lyes or solutions may also be used, so that a higher heating of the residual gases may be obtained for a given addition of water vapor or steam.

It is to be remarked that the consumption of energy of the circulation pump is so slight that it is of little importance in comparison to the advantages obtained.

In the accompanying drawing an apparatus is illustrated suitable for carrying on the process and illustrating diagrammatically the various steps of the process, this drawing following Fig. 1 of Patent 1,954,317 for all common elements.

In the specific embodiment illustrated in the accompanying drawing, air is compressed in a compressor I and ammonia is compressed in a compressor 2, and the mixture may then be ture to, for example, 850 C. The hot gases areY then passed first through the heat exchanger 5 in which they heat the residual gases to be supplied to the compressing apparatus I, and thence through a heat exchanger 3 in which heat is given up to the air and ammonia mixture passing to the combustion chamber. The gases containing the nitric oxide cooled to, for example, 240 C., pass to a condenser 6 in which they are cooled to approximately room temperature. The gases containing the nitrogen oxides then pass through nitric acid absorbers 'I in which these oxides are absorbed in water to form nitric acid, and then pass to an alkaline washer 8 where residual traces of the nitrogen oxides are removed. 'I'he gases instead of passing directly to the heat exchanger 9, as in Patent 1,954,317, pass to a saturator or scrubber I8 which is supplied with water heated in the condenser 6. The water may be supplied to and through the condenser 6 by means of a pump I1. It then enters at the top of the saturator or scrubber I8 and moves downwardly in intimate contact with the residualgases which pass upwardly through the scrubber so that the gases become saturated with steam or moisture. The liquid which falls to the bottom of the scrubber or saturator I8 and has been cooled by the evaporation of water, then passes through an outlet Valve I9 and may pass out of the system. Through the contact of the residual gases with the hot liquid from the condenser 6, the gases are not only heated by the latter, but also are augmented in volume. For example, if they be heated to approximately 90 C., they will be increased in volume by about 10%.

The gases augmented with the steam then pass through the heat exchanger 9 and then through the heat exchanger 5 and to the compression apparatus I to drive the latter, and then return through the heat exchanger 9. The exhaust gases from the latter may be brought into contact with water supplied to the scrubber I8 and the last traces of heat thus recovered. Inasmuch as the volume of the gases has been increased and as provision is made for recovering residual heat from the exhaust gases leaving the heat exchanger 9, the area of the latter may be reduced while maintaining suilicient energy for the compressor I. The usual valves I 2, I3. I5

and I6 and a steam supply pipe for starting the apparatus may be provided, as in the apparatus of Patent 1,954,317.

What I claim is:-

l. In the production of nitric acid under pressure by the oxidation of ammonia by air under pressure, the indirect cooling of the products of combustion by water, absorption of the resulting oxides in water and utilizing the residual gases in an expansion engine to compress air for said combustion, the process which comprises saturating said residual gases from said absorption by direct contact with hot Water used in cooling said products of combustion, heating said residual gases after contact with said water iirst by abstraction of heat from the exhaust gases from said expansion engine and then by abstraction of heat from the products of said oxidation.

Y2. In the production o1' nitric acid under pressure by the oxidation of ammonia by air under pressure, the indirect cooling of said products of combustion by water, absorption of the resulting oxides in water and utilizing the residual gases in an expansion engine to drive a compressor to compress air for said combustion, the process which comprises contacting said residual gases directly with Water heated by absorption oi.' heat from said products o1' combustion, heating the residual gases and the water vapor from said heated water by indirect heat exchange first by abstraction of heat from the exhaust gases from said expansion engine, then by abstraction of heat from the products of said oxidation, and then compressing air for said oxidation by expanding said heated residual gases in the expansion engine.

3. In the production of nitric acid under pressure by the oxidation of ammonia by air under pressure, cooling the products of oxidation, absorption of the resulting oxides in water and utilizing the residual gases in an expansion engine to drive a compressor to compress air for said combustion, the process which comprises heating lWater by direct contact with hot gases in said process, contacting said heated water with residual gases from said absorption to saturate said gases, transferring heat by indirect heat exchange from the exhaust gases from said expansion engine to said saturated residual gases, then transferring heat from the products of combustion to said residual saturated gases, and compressing air for said oxidation by expansion of said heated residual gases in the expansion engine.

4. The process of claim 3 in which the water contains inorganic solutes.

THADEUS HOBLER. 

